Configuring OSPF Priority
When architecting a correct OSPF network, you may need to have specific routers or layer 3 switches be the DR and/or BDR. The OSPF election process is influenced by the OSPF Priority. This lab will discuss and demonstrate the configuration and verification of OSPF Priority.
Real World Application & Core Knowledge
If you completed the previous lab then theirs a high possibility that R1 may not be the designated router and this will most definitely cause a routing update problem.
First off when you configured OSPF on R1 first, it extrapolated the loopback interface IP address as its router-id, which in this case would be 10.90.10.1. When enabling OSPF on R2, it would have got the RID of 10.90.20.1. By default both routers interface priority would have been 1 so the DR/BDR election process tie breaker would have been the router ID, and in this case R2 would have became the Designated Router as it has a higher router ID number and R1 would have became the DBR.
Take a step back and look at the big picture for a minute and lets say R5 needs to send an Update to the DR, in which case it will look up the frame relay map and send the multicast update to 224.0.0.5 out dlci 521. In order to get to the DR (R2) the packet must traverse R1. Now if you think back and remember in Lab 9-1, you should recall that OSPF sends multicast traffic using a TTL of 1, so with this in mind soon as the packet reaches R1, it would not be forwarded to the DR because its TTL would be decremented to 0 and dropped. With this being a problem, routers on the hub and spoke network would not function properly as updates would not properly reach the DR.
To fix this problem you need to configure the spoke routers to NEVER become the DR/BDR. This is done by setting the interface priority to 0. The interface priority is an integer between 0-255 and is configured using the ip ospf priority # command in interface configuration mode.
You could also fix this problem by raising R1’s frame-relay hub and spoke interface priority to 10 for example but this would not prevent R2, R4 and R5 from becoming a BDR, in which case would become the DR in the event of a DR failure thus putting you back at square one.
You can verify which neighbors are the DR/BDR/DROTHER by using the show ip ospf neighbor command in privileged mode.
Familiarize yourself with the following new command(s);
| Command | Description |
|---|---|
| ip ospf priority # | Configures an OSPF priority on a per interface basis used to manipulate the DR/BDR election process. |
The following logical topology shown below is used in labs found through out Section 9 – Configuring OSPF;
Lab Prerequisites
- If you are using GNS3 than load the Free CCNA Workbook GNS3 topology than start devices; R1, R2, R3, R4, R5 and SW1.
- Establish a console session with devices R1, R2, R3, R4, R5 and SW1 than load the initial configurations provided below by copying the config from the textbox and pasting it into the respected routers console.
Lab Objectives
- Configure the Serial interfaces on the Frame relay spoke routers with an OSPF priority to ensure they NEVER become the DR/BDR.
- Clear the OSPF routing process on R1 so that all neighbor relationships are rebuilt then using R2, verify that R1 has became the DR.
Lab Instruction
Objective 1. – Configure the Serial interfaces on the Frame relay spoke routers with an OSPF priority to ensure they NEVER become the DR/BDR.
R2>enable R2#configure terminal Enter configuration commands, one per line. End with CNTL/Z. R2(config)#interface Serial0/0 R2(config-if)#ip ospf priority 0 R2(config-if)#end R2#
R4>enable R4#configure terminal Enter configuration commands, one per line. End with CNTL/Z. R4(config)#interface Serial0/0 R4(config-if)#ip ospf priority 0 R4(config-if)#end R4#
R5>enable R5#configure terminal Enter configuration commands, one per line. End with CNTL/Z. R5(config)#interface Serial0/0 R5(config-if)#ip ospf priority 0 R5(config-if)#end R5#
Objective 2. – Clear the OSPF routing process on R1 so that all neighbor relationships are rebuilt then using R2, verify that R1 has became the DR.
R1#clear ip ospf 1 proc Reset OSPF process? [no]: y R1# %OSPF-5-ADJCHG: Process 1, Nbr 10.90.20.1 on Serial0/0 from FULL to DOWN, Neighbor Down: Interface down or detached %OSPF-5-ADJCHG: Process 1, Nbr 10.90.40.1 on Serial0/0 from FULL to DOWN, Neighbor Down: Interface down or detached %OSPF-5-ADJCHG: Process 1, Nbr 10.90.50.1 on Serial0/0 from FULL to DOWN, Neighbor Down: Interface down or detached R1# %OSPF-5-ADJCHG: Process 1, Nbr 10.90.20.1 on Serial0/0 from LOADING to FULL, Loading Done %OSPF-5-ADJCHG: Process 1, Nbr 10.90.50.1 on Serial0/0 from LOADING to FULL, Loading Done %OSPF-5-ADJCHG: Process 1, Nbr 10.90.40.1 on Serial0/0 from LOADING to FULL, Loading Done R1#
R2#show ip ospf neighbor
Neighbor ID Pri State Dead Time Address Interface
10.90.10.1 1 FULL/DR 00:00:31 10.90.2451 Serial0/0
R2#
